1. Technical Field
The present invention relates to a laser marking method which forms characters or symbols by dots or an arrangement of dots formed by illuminating a laser beam, on a photosensitive material in which a surface layer including an emulsion layer is formed on the surface of a base layer. More specifically, the present invention relates to a laser marking method which forms a marking pattern such as, for example, characters or symbols, by illuminating a laser beam onto a photosensitive material which may be a light-photosensitive heat-developing photosensitive material.
2. Description of the Related Art
In medical fields in recent years, a reduction in waste solutions from processing arising when developing processing of X-ray films is carried out, has been desired from the standpoints of preserving the environment and reducing the space required for devices. Moreover, there have been provided light-photosensitive heat-developing photographic materials (light-photosensitive heat-developing photosensitive materials) for medical diagnoses and photographic technological applications which can be effectively exposed by using a laser image setter or a laser imager, and which can form a clear image having high resolution and good sharpness. In this way, in medical fields as well, attention has focused on heat-developing processing systems using such light-photosensitive heat-developing photosensitive materials.
Here, there is a marking technique which forms characters, symbols or the like by dots or an arrangement of dots formed by illuminating a laser beam onto a photosensitive material such as an X-ray film such that heat-induced fog or deformation occurs at the surface of the photosensitive material. As a condition for generating heat-induced fog or deformation which has good visibility by using such a marking technique, the illuminating of a laser beam over a relatively long period of time by using a laser oscillator of a low output of 50 W or less has been proposed. For example, refer to Japanese Patent No. 3191201.
In a so-called dry film such as an X-ray film using a light-photosensitive heat-developing photosensitive material, at the base layer which is the support, the transparency of the surface layer including the emulsion layer is of course high.
Here, in order to form a dot having high visibility by using a low-output laser oscillator, the period of time over which the laser beam is illuminated is long. Thus, at the film surface, the surface layer melts and opens up in a crater-like form, such that the PET forming the base layer is exposed.
Therefore, at a highly-transparent dry film, the borders between regions at which the laser beam is not illuminated and the portions where the laser beam is illuminated and openings are formed, are not distinct. It is difficult to form dots which have sufficient visibility.
In order to avoid such a decrease in visibility, there has been proposed the use of a film in which an indication portion, which uses a dye or a pigment for indication of a pattern, is provided in advance at the film surface. Refer to, for example, Japanese Patent No. 2829780.
However, the existence of a pigment or a dye at the surface of the photosensitive material may affect the image formed on the photosensitive material. Further, in order to avoid such effects, further cost and efforts are required.
In photosensitive materials such as X-ray films for medical use, characters for example are recorded on the edge portion in order for the manufacturer, the product type, and the lot to be able to be identified. A marking technique is known in which, when characters, symbols or the like are to be recorded on a photosensitive material such as an X-ray film, laser light (a laser beam) is illuminated onto the photosensitive material, and dots are formed by heat-induced fog or deformation arising at the surface of the photosensitive material, and a marking pattern of, for example, characters or symbols, is formed by an arrangement of these dots.
As shown in FIGS. 22A and 22B for example, in an X-ray film 190, by illuminating a laser beam, melting and transpiration take place at the emulsion surface due to the energy of the laser beam. In this process, a large number of minute air bubbles arise at the interior of an emulsion layer 192 which swells, and a dot is formed.
As shown in FIG. 22A, in a highly-visible dot 194, the surface projects (is convex) due to the large number of air bubbles arising in the emulsion layer 192. The irregular reflection of the light at the border films between the large number of minute air bubbles is promoted, and becomes great in the large change in the reflected light amounts at the interior and the exterior of the dot 194.
When characters or symbols are formed by an arrangement of such dots, the dot diameter and the interval between dots must be set appropriately. Further, high visibility, i.e., a good finished quality, is required of each of the individual dots 194.
At this time, for example, when energy of an amount greater than needed is applied to the emulsion layer 192 by a laser beam, as shown in FIG. 22B, the emulsion layer 192 melts and opens, such that a dot 198, which exposes a base layer 196 which is the support, is formed.
In the case of the X-ray film 190 at which the emulsion layer 192 has high transmittance, it is not possible to visually perceive the dot 198 because it is difficult to differentiate between the emulsion layer 192 and the exposed based layer 196. Namely, the visibility of the dot 198 is extremely low, and the visibility of characters, symbols or the like formed by a dot arrangement including these dots 198 also is extremely low.
Accordingly, when forming a dot having high visibility (the dot 194) on an X-ray film by using a laser beam, the time over which the laser beam is illuminated onto the X-ray film is controlled appropriately such that proper deformation is made to occur at the X-ray film due to the energy of the laser beam.
By increasing the visibility of the individual dots in this way, the visibility of the marking pattern formed by the arrangement of dots is improved.
In a laser oscillating tube which oscillates laser light, the output peak immediately after driving begins is high. By continuing driving, the output gradually decreases. After a predetermined time elapses, the output is stabilized and the state of steady output is reached.
When forming a character by a dot arrangement or a marking pattern in which a plurality of characters are consecutively recorded by such a laser oscillating tube, the visibility of the dots or the characters formed by dot arrangements, which are formed immediately after marking begins, is extremely poor. Namely, when the laser beam is illuminated onto the photosensitive material in the state in which the output peak is high, the melting and transpiration of the emulsion layer progresses such that the base layer is exposed.
As a method of preventing the effects of the output peak which occurs immediately after the start of driving of the laser oscillating tube when marking is carried out by using a laser beam, there has been proposed a method of carrying out marking in which, when marking is carried out by pulse-driving a laser oscillating tube, driving of the laser oscillating tube is started in a state in which a shutter is closed, and thereafter, in a state in which the shutter is opened, actual marking by pulse-driving is carried out. See, for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-52069.
In this way, the marking is not affected by the output peak which arises immediately after the start of driving of the laser oscillating tube, and marking using a laser beam is possible.
However, when continuously pulse-driving a laser oscillating tube, the output peak arises immediately after the start of driving. Further, at the time of driving by each driving pulse as well, first, an output peak is exhibited, and then the output gradually falls toward the output in a steady state.
Namely, when forming one character or a character array, when a laser oscillating tube is driven continuously, as shown in FIG. 23A, the output of the laser beam is extremely high immediately after the start of driving. As time passes, the output gradually decreases and is stabilized in a steady state. In contrast, when a laser oscillating tube is pulse-driven, as shown in FIG. 23B, the output peak of the laser oscillating tube which is driven by respective pulses gradually decreases. However, during the time of driving by each pulse, a variation in output occurs such that the output is highest immediately after the start of driving, and then the output suddenly falls.
When dots are formed at a photosensitive material, such a variation in output of the laser beam which is illuminated from the laser oscillating tube may result in the energy of the laser beam concentrating at one portion.
In this way, the progression of the melting and transpiration of the emulsion layer occurs and the base layer is exposed at, for example, the central portion of the dot or the like, such that deformation of the dot, a decrease in the diameter of the dot, or the like occurs. When such dots are formed, the visibility of the dots and of characters, symbols, and the like formed by an arrangement of the dots deteriorates.